1. Field of the Invention
In the processing of polymer materials the material to be processed is put into a fluid state either by the application of heat or by dissolving it in a low molecular weight solvent to form a solution. The resulting fluid is of a viscoelastic nature and may be formed into a fabricated shape either by flow under the force of gravity or by flow under an externally applied pressure.
In order to properly design such processing operations and the equipment associated therewith it is important to have measurements of the rheological behavior of the viscoelastic polymer melt or solution. Since these materials are viscoelastic, it is important and necessary to measure both the viscous and elastic behavior of the material.
There are well established methods of measuring the viscous properties, for example the standard melt index test (American Society for Testing and Materials, D-1238). Methods of measuring the elastic properties are not as well established and no satisfactory method is as yet available. The present invention is intended to fill this need.
2. Description of the Prior Art
Current methods of measuring the elastic properties of polymer melts and solutions include: (1) The Orthogonal Rheometer as first disclosed by Bryce Maxwell and Richard P. Chartoff in Transactions of the Society of Rheology 9:1, 41-52 (1965). This method measures the dynamic elastic modulus but cannot measure the elastic recoverable strain, (2) The well known Couette apparatus and the Searle apparatus (see for example, Rheology Vol. 3 pg. 29, Academic Press, N.Y., N.Y.) Both the Couette and Searle methods involve shearing the material specimen in an annular space between a cylindrical center member and an outer cup member and measuring the force required to achieve the shearing as a function of the rate of shear. The present invention discloses an improvement of this type of apparatus usable to measure the elastic recoverable strain.
Past efforts in devising apparatus for the measurement of recoverable strain have resulted in complex and expensive systems both in the apparatus and in the reduction of the data generated. For example Maxwell and McCord (Modern Plastics Magazine, Sept. 1961) resorted to a system involving a motion picture camera photographing the motion of a lever attached to the central cylindrical member simultaneously with a stop watch to record the strain recovery as a function of time. Such a system is complex in the apparatus required and time consumed in the delay required to develop the motion picture film and also tedious in the frame by frame analysis of photographic recording.
The present invention overcomes many of these undesirable features due to the simple manner in which the data is immediately available.